The present invention relates to an accumulator for absorbing pressure pulsation and accumulating pressure in various forms of vehicle onboard equipment, such as hydraulic brakes for vehicles, and industrial hydraulic machinery.
Accumulators having a pressure vessel divided into two chambers by a bellows to fill one of the chambers with high pressure gas are known. When such an accumulator is exposed to a high temperature for a prolonged period of time as is the case in a fire, the inner pressure rises sharply with the rise in temperature, and the heat may damage the pressure withstanding strength to such an extent that the pressure vessel could rupture.
To avoid such a problem, it is proposed in Japanese UM publication (kokoku) 06-6241 to provide an exhaust opening in the pressure vessel to communicate the gas chamber with the atmosphere and insert a fuse packing that melts at a prescribed temperature in this exhaust opening.
According to such a safety measure, the sealing of the pressure vessel in normal use must rely on the sealing capability of the fuse packing which however is unable to provide an adequate sealing capability. Therefore, according to such an arrangement, it would be difficult, if not impossible, to prevent leakage of gas from the exhaust opening and ensure a required pressure level over an extended period of time.
In view of such problems of the prior art, a primary object of the present invention is to provide an accumulator that can safely release the high pressure gas in case of a fire.
A second object of the present invention is to provide an accumulator incorporated with a safety valve that is capable of releasing the high pressure gas when required without reducing the sealing capability during normal use.
A third object of the present invention is to provide an accumulator incorporated with a safety valve that is simple and economical.
According to the present invention, such objects can be accomplished by providing an accumulator, comprising: a pressure vessel having a liquid introduction port at an end thereof; a bellows having a first end attached to the inner surface of the pressure vessel so as to separate the interior of the pressure vessel into a liquid chamber communicating with the liquid introduction port and a gas chamber filled with pressurized gas; a stay member attached to the inner surface of the pressure vessel inside the liquid chamber defining therein a small chamber communicating with the liquid introduction port, the stay member being provided with a communication port for communicating the interior of the small chamber with the remaining part of the liquid chamber and adapted to be closed by a part of the bellows to define a fully contracted state of the bellows; and a safety valve provided in the stay member for selectively communicating the exterior of the stay member with the interior of the stay member.
Thus, because the safety valve is not provided in the pressure vessel itself, and because the safety valve is provided in the stay member which is entirely received in the liquid chamber and is not subjected to any pressure difference during normal use, the safety valve of the present invention would not affect the sealing capability of the pressure vessel.
Also, the safety valve is normally immersed in liquid and liquid can be sealed more easily than gas. Furthermore, even when there is a small leakage in the safety valve, it would not cause any serious problem.
The bellows may consist of a substantially cylindrical bellows having a first axial end attached to the inner surface of the end of the pressure vessel and a second axial end closed by an end plate, the end plate being adapted to close the communication port in the fully contracted state of the bellows.
According to a preferred embodiment of the present invention, the stay member comprises a cup-shaped member having an open end attached to the periphery of the liquid introduction port inside the pressure vessel, a cylindrical side wall and a free end closed by an end plate having the communication port formed therein, and the bellows consists of a metallic bellows. In such a case, the safety valve may be conveniently placed in the side wall of the stay member.
The safety valve may consist of a common mechanical valve including a valve seat communicating the interior of the stay member with the exterior of the stay member and a spring-loaded valve element normally closing the valve seat. However, it may also consist of a weakened portion of the stay member so that the structure may be simplified and the manufacturing cost may be minimized. Typically, the weakened portion comprises a localized thin-walled portion of the stay member.
The weakened portion may also consist of an annular groove extending entirely around the side wall. In this case, the annular groove may be located on a plane perpendicular to the axial line of the stay member or on a plane at an angle with respect to an axial line of the stay member. If the annular groove is located on a plane at an angle with respect to an axial line of the stay member, when the inner pressure of the gas chamber rises excessively and the annular groove breaks, the opposing annular edges of the two parts of the stay member above and below the annular groove abut each other on this angled plane, and this causes a lateral movement of the upper part of the stay member. Therefore, the communication port is prevented from being closed, and the required relief action can be ensured without fail.
According to another preferred embodiment of the present invention, the safety valve comprises a hole formed in the stay member and a plug fitted into the hole and made of material having a relatively low melting point. This melting point is selected to be at least lower than that of the material of the pressure vessel so that the plug melts away and permits the gas chamber to be communicated with the liquid pressure circuit via this hole and liquid introduction port before the pressure vessel ruptures in case of a high temperature situation such as a fire.